Vibration damped sandwich systems



DeC. 15,1970 Q ETAL 3,547,759

7 VIBRA'IfIGN DAMPED SANDWICH SYSTEMS Filed Nov. 18-, 1968 21 Sheets-Sheet 1 0.5 comb d c omb INVENTORS BY 7 w;

ATTORNEYS Dec. 15,1970 -r QETAL 7 3,547,759

' VIBRATION DAMPED SANDWICH SYSTEMS Filed Nov 18. 1968 2 Sheets-Sheet 2 HERMANN cree er JOACHIMV EBIGT summer; DUVE ALFRED lBCHOMMEEF- ATTORNEYES United States Patent 3,547,759 VIBRATION DAMPED SANDWICH SYSTEMS Hermann Oberst, Hofheim, Taunus, Joachim Ebigt and Giinther Duve, Frankfurt am Main, and Alfred Schommer, Bad Soden, Taunus, Germany, assignors to Farbwerke Hoechst Aktiengesellschaft vormals Meister Lucius & Bruning, Frankfurt am Main, Germany, a corporation of Germany Filed Nov. 18, 1968, Ser. No. 776,633 'Claims priority, application Germany, Dec. 5, 1967, 1,700,120 Int. Cl. B32b 15/08; C08f 15/20; F0412 1/99 U.S. Cl. 161165 4 Claims ABSTRACT OF THE DISCLOSURE A vibration damped sandwich system comprising two hard plates and interposed between the plates a vibration damping interlayer comprising a graft polymer of methylmethacrylate or optionally methylmethacrylate together with small amounts of a copolymerizable carboxylic acid, especially acrylic acid and/or methacrylic acid, on copolymers of 30 to 40% by weight of vinyl acetate, 30- 40% by weight of n-butyl acrylate, 30 to 10% by weight of dibutyl maleate and 5 to by weight of crotonic acid.

The present invention relates to vibration damped sandwich systems having interlayers made of graft polymers of methylmethacrylate or optionally methylmethacrylate together with small amounts of a copolymerizable carboxylic acid, especially acrylic acid and/or methacrylic acid, on vinyl acetate/n-butyl acrylate/dibutyl maleate/ crotonic acid copolymers.

It is known from South African specification No.

5,269 that highly valuable vibration damping materials of a broad temperature band suitable for damping bending vibrations of metal sheet constructions can be prepared by copolymerizing monomers whose homopolymers differ in their second order transition temperature by at least C. The above specification also reports that as vibration damping materials having a broad temperature hand there can be used, among others, predominantly amorphous copolymers of esters of alcohols with 4 to 12 carbon atoms with acrylic and maleic acid and vinyl esters of fatty acids with 2 to 3 carbon atoms, for example vinyl acetate/n-butyl acrylate and/ or dibutyl maleate copolymers.

It has now been found that graft polymers of methylmethacrylate or optionally methylmethacrylate together with small amounts of a copolymerizable carboxylic acid, especially acrylic acid and/or methacrylic acid, on copolymers of vinyl acetate, suitable esters of unsaturated polymerizable carboxylic acids and an unsaturated copolymerizable acid, preferably crotonic acid have outstanding damping properties and a very broad temperature range of damping and are especially suitable for the vibration damping of sandwich systems of hard plates, in particular metal sheets. Suitable esters are especially those of acrylic acid and maleic acid with alcohol components having 3 to 12 carbon atoms, preferably n-butyl acrylate and dibutyl maleate. With these graft polymers, the vibration damping effect of which critically depends on the Weight proportion of the monomers, very broad damping curves can be obtained having very high maximum damping values. From the economical point of view they have the advantage of being rather cheap.

The graft polymers of methylmethacrylate or optionally methylmethacrylate together with a copolymerizable carboxylic acid (acrylic and/or methacrylic acid) on copolymers of to by weight of vinyl acetate, 30

to 40% by weight of n-butyl acrylate, 30 to 10% by weight of dibutyl maleate and 5 to 15% by weight of crotonic acid are prepared by free radical initiated p0- lymerization at a temperature in the range of from about to about 180 C. of a solution of the copolymer in monomeric methylmethacrylate or a mixture of methylmethacrylate and a copolymerizable carboxylic acid (acrylic and/or methacrylic acid) which solution contains the catalyst. Especially good results are obtained with graft polymers of 40 to 80% by weight of methylmethacrylate or a mixture of methylmethacrylate with a copolymerizable carboxylic acid (acrylic acid and/or methacrylic acid) on 60 to 20% by weight of the copolymers specified above, for example a graft polymer of 50% by weight of methylmethacrylate or of 50% by weight of a mixture of 90% by weight of methylmethacrylate and 10% by weight of acrylic acid and/or methacrylic acid on 50% by weight of the aforesaid copolymer. As initiator for the polymerization tertiary butylhydroperoxide may be used in the usual concentration.

The present invention provides sandwich systems of hard plates, in particular metal sheets, having vibration damping, self-adherent interlayers of graft polymers of methylmethacrylate or optionally methylmethacrylate together with small amounts of a copolymerizable carboxylic acid especially acrylic acid and/or methacrylic acid, on vinyl acetate/n-butyl acrylate/dibutyl maleate/ crotonic acid copolymers, for which interlayers there are used graft polymers of 40 to 80% by weight of methylmethacrylate or optionally methylmethacrylate together with small amounts of a copolymerizable carboxylic acid (acrylic acid and/or methacrylic acid) on 60 to 20% by weight of a copolymer consisting of 30 to 40% by weight of vinyl acetate, 30 to 40% by weight of n-butyl acrylate, 30 to 10% by weight of dibutyl maleate and 5 to 15 by weight of crotonic acid.

FIGS. 1a and 1b of the accompanying drawings are plots against temperature of the loss factor dcomb to illustrate the superior efficiency of the novel systems. The curve in FIG. 1a shows the loss factor dcomb of a metal sheet arrangement of the invention as a function of temperature. For comparison, one of the most effective vibration damping materials known for metal sheet arrangements was used, namely a copolymer of 63% by weight of vinyl acetate and 37% by weight of dibutyl maleate containing as plasticizer 15% by weight of 2- ethylhexyl phthalate and 15% by weight of tricresyl phosphate, calculated on the mixture (curve FIG. 1b). The copolymer of curve 1b was a thermoplastic adhesive especially suitable for producing vibration damped metal sheet sandwitch systems comprising two outer metal sheets and a self-adherent thermoplast as damping interlayer. Systems of this type provide a damping effect which is extremely high in its maximum and cannot be exceeded for physical reasons (cf. H. Oberst and A. Schommer, Kunststoffe, vol. 55, p. 634 1965), especially FIG. 9).

In a symmetrical arrangement comprising two metal sheets, each having a thickness of 0.5 millimeter, and an interlayer 0.3 millimeter thick, the loss factor dcomb of the combined system, measured in the bending wave method (cf. for example H. Oberst, L. Bohn and F. Linhardt, Kunststotfe, vol. 51, p. 495 (1961)), almost reaches the value dcomb of 1. The known metal sheet damping by one-side damping coatings which are applied by spraying, trowelling or bonding in the form of layers of so-called vibration damping materials show loss factors generally of less than d =02 with technically reasonable thicknesses or ratios of coating mass to plate mass of the combined system. With metal sheet sandwich systems which gain growing importance in recent times, it is possible to obtain damping values that are increased by 3 a multiple, as shown by the present example, when the interlayer material has the appropriate composition and thickness.

The temperature band width of the damping of the metal sheet sandwich system does not only depend on the viscoelastic properties of the interlayer and the steel sheets but also to a considerable extent on the geometry of the arrangement i.e. on the ratio of the layer thicknesses (cf. loc. cit (1965), FIGS. 8 to 10). With metal sheet sandwich systems the band width is advantageoulsy defined as the range of the temperature interval within which the value d =r0fi is exceeded. The damping of metal sheets which are not damped by additional vibration damping means in metal sheet constructions of various types corresponds to values d 0.01. The reference value dcomb of 0.05 thus means a considerable increase in the damping effect (by about 15 db (decibel)) as compared to the nil damping d =0.01. 3

In the curve shown in FIG. 1b the reference value dcomb of 0.05 is exceeded in the mainly interesting frequency range of from 100 to 1,000 c.p.s. (Hz.) at temperatures ranging from about 0 to 50 C. The temperature band width thus corresponds to about C. Sandwich systems of this type are suitable for many technical fields of application. By modifying the content of plasticizer, it is possible to shift the temperature band of a high damping effect to higher temperatures and thus to adapt the material to special technical uses, for example in machine units operating at elevated temperatures. This example of a metal sheet sandwich system comprising a self-adherent interlayer having optimum properties of a vibration damping material with a broad temperature band prepared by copolymerizing appropriate monomeric compounds has hitherto not been surpassed by other arrangements of similar kind and may be taken as standard for judging the acoustic efficiency of the system according to the invention.

FIGS. 1a and 111 show the temperature curves of the loss factor dcomb of metal sheet sandwich systems comprising steel sheets of a thickness of 0.5 millimeter each and damping interlayers of a thickness of about 1 and 0.3 millimeter, respectively, (FIG. 12) for a frequency of 100 c.p.s. and 1,000 c.p.s.

The curves were measured with sandwich systems the intrelayers of which consisted of (1a) A graft polymer of 50% by weight of a mixture of 90% by weight of methylmethacrylate and by weight of acrylic acid on 50% by weight of a copolymer of 35% by weight of vinyl acetate, 35% by weight of n-butyl acrylate, 20% by weight of dibutyl maleate and 10% by weight of crotonic acid (according to the invention) (1b) A copolymer of 60% by weight of vinyl acetate and 37% of dibutyl maleate containing as plasticizer 15% by weight of Z-ethylhexyl phthalate and 15% by weight of tricresyl phosphate, calculated on the mixture.

The arrangement 1a, whose monomer proportion lies in the optimum range has a very broad temperature band width with relatively high maximum damping values. With the arrangement 1a the center of damping is at about 50 to about 65 C. with maximum damping values of 0.4 to about 0.9. The temperature band width is about 155 C. for 100 c.p.s. and about 125 C. for 1,000 c.p.s. In arrangement 1a, the excellent vibration damping properties are maintained, calculated on d =0.05, from about 0 C. to about 135 C. The slow decrease of damping towards low temperatures and particularly towards high temperatures, especially at low frequencies of about 100 c.p.s., is very favorable.

As compared with the standard system 1b, arrangement 111 has an appreciably broader temperature band width and better vibration damping properties at temperatures above 35 C. so that arrangements of this type are suitable for quite a number of applications (for example in machine units and appliances operated at normal and high temperatures). Owing to its content of 10% by weight of crotonic acid and acrylic acid the graft polymer used in arrangement la can be cross-linked by reaction with a bifunctional or trifunctional compound (for example a compound containing a plurality of epoxide, isocyanate or similar groups), whereby the softening range and therewith the range of high damping can be noticeably shifted towards higher temperatures.

A special advantage of the vibration damping material of the present invention resides in the fact that it may be applied continuously during the mass production of the metal sheet sandwich systems.

For this purpose the vibration damping material can be used (1) in the form of the finished graft polymer and (2) in the form of a solution of the specified copolymer in the methylmethacrylate or methylmethacrylate/ acrylic and/or methacrylic acid mixture to be grafted on the copolymer and containing the polymerization catalyst, and the graft polymer can be produced by a thermal treatment at a temperature in the range of from about 60 to about 180 C. of the sandwich system.

It is a thermoplastic adhesive which may be applied to the metal sheets by trowelling, brushing or pouring at elevated temperature. The sandwich system may then be advantageously cooled under pressure between rollers. Except for degreasing the metal sheets do not require a preliminary treatment and further adhesive. Owing to the content of the graft polymer of crotonic acid and optionally acrylic acid and/ or methacrylic acid degreasing may even be dispensed with. The adhesion is very good.

The vibration damping material of the present invention has a good resistance to flow. The metal sheet sandwich system may, within broad limits, be processed as normal metal sheets, that is they may be creased, bent, shaped, welded and riveted.

In this manner laminated systems are obtained having a damping height and a temperature range of damping which makes them well suitable for many applications, especially at normal and elevated temperatures.

Minor amounts of fillers, for example for improving the electric conductivity (improvement of resistance welding) may be incorporated in the vibration damping materials. In order not to affect the damping effect adversely it is advantageous to use less than 1% by weight, preferably less than 0.5% by weight of filler, calculated on the polymer. Suitable fillers are, for example, heavy spar, silicic acid, graphite and soot.

The metal sheet sandwich system suitably has a total thickness in the range of from 1 to 6 millimeters. The interlayers may have a thickness of 0.1 to 1 millimeter, preferably 0.2 to 0.5 millimeter. A maximum damping effect is obtained with symmetrical laminated systems. With an equal weight, asymmetrical laminated systems have, however, a higher stiffness in flexure and strength. Asymmetrical laminated systems are, therefore, preferred for those applications which require a high strength with respect to the weight. The ratio of the thicknesses of the outer plates or metal sheets is preferably within the range of from 1:1 to 1:4.

FIG. 2 of the accompanying drawings shows sandwich systems with symmetrical arrangement (a) and asymmetrical arrangement (b) in which the interlayer 2 is interposed between the two outer plates or metal sheets 1.

What is claimed is:

1. A vibration damped sandwich system comprising two hard plates and interposed between the plates a vibration damping interlayer comprising a graft polymer of 40 to by weight of methylmethacrylate or a mixture of methylmethacrylate with 0.1 to 10% by weight of acrylic acid, methacrylic acid, or mixtures thereof calculated on the methylmethacrylate, on 60 to 20% by weight of a copolymer of 30 to 40% by weight of vinyl acetate, 30-40% by weight of n-butyl acrylate, 30 to 10% by weight of dibutyl maleate and 5 to 15% by weight of crotonic acid.

2. A vibration damped sandwich system as claimed in 3,399,103 8/1968 Salyer et a1. claim 1, wherein the hard plates are metal sheets.

3. A vibration damped sandwich system as claimed in FOREIGN PTENTS claim 1, wherein the graft polymer of the interlayei" con- 245,224 Austral?- tains up to 1% by weight of a filler, calculated on the graft 5 245,685 1/ 1963 Australlapolymer.

4. A vibration damped sandwich system as claimed in JOHN GooLKslAN Primary Exammer claim 1, wherein the ratio of the thicknesses of the 'plates G, W, MOXON II', A i ta t Examiner lies in the range of from 1:1 to 1:4."

1 US. Cl. X.lR.

References .Cited UNITED STATES PATENTS 3,271,188 9/1966 Albert et a1. 

